A charge amplifier may be driven by an electrical signal source with capacitive nature such as a piezoelectric sensor, or a charge-coupled device (CCD). The charge amplifier may transfer the charge from a charge signal to a reference capacitor (also referred to as an integrating capacitor) to generate an output voltage that correlates to the voltage across the reference capacitor. The output voltage may be based on the charge of the reference capacitor and on the input charge. Charge amplifiers may be used for application in the readout circuits of optical imaging devices and flat-panel X-ray detector arrays, to measure small charge stored in small capacitors of individual pixels in arrays of pixels.
In multiple-input charge based circuits, selection circuit or multiplexor (MUX) may be used to channel specific sensing capacitors to the charge amplifier for signal processing. The input charge signal, which may be stored in a capacitor for each pixel, may be much less than the error charge induced by parasitic capacitance of the thin film transistor (TFT) selection switch during a transition of the selection switch. As a result, when a pixel is selected, the smaller input charge signal of the pixel may be added to the larger selection-induced error charge, and may cause the charge amplifier to saturate early, thus limiting its dynamic range, or may cause the automatic gain selection circuit to select a lower gain early, thus limiting full advantage of the allowable dynamic range available to sense or amplify the smaller input charge signal. When the selection switch is turned off, the charge amplifier should only represent the input charge signal, but the larger error charge induced by the selection switch may corrupt the output voltage signal.
Thus, there is a need for an improved charge amplifier that compensates for errors without significantly increasing device complexity and size or significantly decreasing performance capabilities.